Resistance welding involves a process for joining metallic work pieces by passing electrical current therethrough. The current flow heats the work pieces and forms a molten "weld nugget". Upon termination of the welding current, the weld nugget solidifies to form the weld.
Various parameters which affect the quality and nature of the weld are controlled by a weld controller. The predominate type of weld controller presently in use utilizes a supply of alternating current (AC) electrical energy. Electrical transformers connected with the weld control function to convert the source of AC power to a relatively low voltage, with relatively high current output on the transformer secondary. The specific levels of current are often mapped out for various work piece materials, as well as for types of different types of joints such as spots, seams or butts. The weld controller is used to regulate the current to the primary of the transformer by means of an electronic switch, typically a silicon controlled rectifier (SCR). In order to accommodate alternating power, two SCRs are installed in inverse relationship, in parallel to handle the positive and negative half cycles, respectively.
In certain applications, the amount of inductance in the secondary loop of the welding transformer can be quite large and prevents achieving the necessary levels of welding current. In order to overcome this problem, the frequency in the secondary of the transformer is reduced by effectively converting the current flowing in the secondary loop to a DC current. In many cases, however, existing methods of compensating for the variations in the power factor and other electrical parameters of a given welding circuit, in order to achieve constant current control, do not perform well when applied to single phase DC welders.
There is therefore a clear need in the art for a weld control method that eliminates the above mentioned problem, and which is suitable for use with either an AC or a DC type resistance welder.